automount-support.rst (3604B)
1.. SPDX-License-Identifier: GPL-2.0 2 3================= 4Automount Support 5================= 6 7 8Support is available for filesystems that wish to do automounting 9support (such as kAFS which can be found in fs/afs/ and NFS in 10fs/nfs/). This facility includes allowing in-kernel mounts to be 11performed and mountpoint degradation to be requested. The latter can 12also be requested by userspace. 13 14 15In-Kernel Automounting 16====================== 17 18See section "Mount Traps" of Documentation/filesystems/autofs.rst 19 20Then from userspace, you can just do something like:: 21 22 [root@andromeda root]# mount -t afs \#root.afs. /afs 23 [root@andromeda root]# ls /afs 24 asd cambridge cambridge.redhat.com grand.central.org 25 [root@andromeda root]# ls /afs/cambridge 26 afsdoc 27 [root@andromeda root]# ls /afs/cambridge/afsdoc/ 28 ChangeLog html LICENSE pdf RELNOTES-1.2.2 29 30And then if you look in the mountpoint catalogue, you'll see something like:: 31 32 [root@andromeda root]# cat /proc/mounts 33 ... 34 #root.afs. /afs afs rw 0 0 35 #root.cell. /afs/cambridge.redhat.com afs rw 0 0 36 #afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0 37 38 39Automatic Mountpoint Expiry 40=========================== 41 42Automatic expiration of mountpoints is easy, provided you've mounted the 43mountpoint to be expired in the automounting procedure outlined separately. 44 45To do expiration, you need to follow these steps: 46 47 (1) Create at least one list off which the vfsmounts to be expired can be 48 hung. 49 50 (2) When a new mountpoint is created in the ->d_automount method, add 51 the mnt to the list using mnt_set_expiry():: 52 53 mnt_set_expiry(newmnt, &afs_vfsmounts); 54 55 (3) When you want mountpoints to be expired, call mark_mounts_for_expiry() 56 with a pointer to this list. This will process the list, marking every 57 vfsmount thereon for potential expiry on the next call. 58 59 If a vfsmount was already flagged for expiry, and if its usage count is 1 60 (it's only referenced by its parent vfsmount), then it will be deleted 61 from the namespace and thrown away (effectively unmounted). 62 63 It may prove simplest to simply call this at regular intervals, using 64 some sort of timed event to drive it. 65 66The expiration flag is cleared by calls to mntput. This means that expiration 67will only happen on the second expiration request after the last time the 68mountpoint was accessed. 69 70If a mountpoint is moved, it gets removed from the expiration list. If a bind 71mount is made on an expirable mount, the new vfsmount will not be on the 72expiration list and will not expire. 73 74If a namespace is copied, all mountpoints contained therein will be copied, 75and the copies of those that are on an expiration list will be added to the 76same expiration list. 77 78 79Userspace Driven Expiry 80======================= 81 82As an alternative, it is possible for userspace to request expiry of any 83mountpoint (though some will be rejected - the current process's idea of the 84rootfs for example). It does this by passing the MNT_EXPIRE flag to 85umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH. 86 87If the mountpoint in question is in referenced by something other than 88umount() or its parent mountpoint, an EBUSY error will be returned and the 89mountpoint will not be marked for expiration or unmounted. 90 91If the mountpoint was not already marked for expiry at that time, an EAGAIN 92error will be given and it won't be unmounted. 93 94Otherwise if it was already marked and it wasn't referenced, unmounting will 95take place as usual. 96 97Again, the expiration flag is cleared every time anything other than umount() 98looks at a mountpoint.